This invention relates to the preparation of nonionic alkanol alkoxylates as the reaction products of alkylene oxides with detergent-range, i.e., C.sub.8 to C.sub.20, alkanols. More particularly, this invention is directed to a process for preparation of such surfactant materials utilizing an alkoxylation catalyst which combines an aluminum compound with a sulfur-containing acid.
A large variety of products useful, for instance, as nonionic surfactants, wetting and emulsifying agents, solvents, and chemical intermediates, are prepared by the addition reaction (alkoxylation reaction) of alkylene oxides with organic compounds having one or more active hydrogen atoms. As an example, particular mention may be made of the alkanol ethoxylates and alkyl-substituted phenol ethoxylates prepared by the reaction of ethylene oxide with aliphatic alcohols or substitued phenols of about 6 to 30 carbon atoms. Such ethoxylates, and to a lesser extent corresponding propoxylates and compounds containing mixed oxyethylene and oxypropylene groups, are most commonly applied as nonionic detergent components of commercial cleaning formulations for use in industry and in the home.
An illustration of the preparation of an alkanol ethoxylate (represented by formula III below) by addition of a number (n) of ethylene oxide molecules (formula II) to a single alkanol molecule (formula I) is presented by the equation. ##STR1##
The addition of alkylene oxides to alkanols and other active-hydrogen containing compounds is known to be desirably promoted by a catalyst, most conventionally a catalyst of either strongly acidic or strongly basic character. Recognized in the art as suitable basic catalysts are the basic salts of the alkali metals of Group I of the Periodic Table, e.g., sodium, potassium, rubidium, and cesium, and the basic salts of certain of the alkaline earth metals of Group II of the Periodic Table, e.g., calcium, strontium, barium and in some cases magnesium. Conventional acidic alkoxylation catalysts include, broadly, the Lewis acid or Friedel-Crafts catalysts. Specific examples of these catalysts are the fluorides, chlorides, and bromides of boron, antimony, tungsten, iron, nickel, zinc, tin, aluminum, titanium and molybdenum. The use of complexes of such halides with, for example, alcohols, ethers, carboxylic acids, and amines have also been reported. Still other examples of known acidic alkoxylation catalyts are sulfuric and phosphoric acids; perchloric acid and the perchlorates of magnesium, calcium, manganese, nickel and zinc; metals oxalates, sulfates, phosphates, carboxylates and acetates; alkali metal fluoroborates, zinc titanate; and metal salts of benzene sulfonic acid.
In one important aspect, the present invention relates to an alkoxylation reaction catalyzed by a particular combination of one or more of certain aluminum compounds with a sulfur-containing acid such as sulfuric acid, sulfur trioxide, or a sulfonic acid. In another important aspect, the invention further involves the discovery of a process for the production of alkylene oxide adducts of alkanols (termed alkanol alkoxylates or simply alkoxylates for purposes of describing this invention) which are characterized by a narrow or peaked alkylene oxide adduct distribution. Alkylene oxide addition reactions are known to produce a product mixture of various alkoxylate molecules having different numbers of alkylene oxide adducts (oxyalkylene adducts), e.g., having different values for the adduct number n in formula III above. The adduct number is a factor which in many respects controls the properties of the alkoxylate molecule, and efforts are made to tailor the average adduct number of a product and/or the distribution of adduct numbers within a product to the product's intended service.
The present invention provides a process characterized by enhanced selectivity for the preparation of alkoxylate mixtures in which a relatively large proportion of the alkoxylate molecules have a number (n) of alkylene oxide adducts that is within a relatively narrow range of values. It has been reported that alkoxylate products having such a narrow range distribution are preferred for use in certain detergent formulations (Great Britain Pat. No. 1,462,134; Derwent Publications Research Disclosure No. 194,010). Narrow-range alkoxylates are also known to be particularly valuable as chemical intermediates in the synthesis of certain carboxyalkylated alkyl polyethers (U.S. Pat. No. 4,098,818) and of certain alkyl ether sulfates (Great Britain Pat. No. 1,553,561). Conventional commercial alkoxylate preparation, which has in large part been limited to the use of basic catalysts, particularly the metals sodium and potassium and their oxides and hydroxides, yields only a relatively broad distribution range product. Conventional acid-catalyzed alkoxylation reactions have long been known to produce a more narrow range product than that obtained with the alkali metal catalysts. Characteristic of the product of the typical acid-catalyzed alkoxylation is a statistical Poisson distribution in which the relative concentration of each individual alkoxylate species may be expressed in terms of the following equation, which is well known to those in the oligomerization and polymerization arts: ##EQU1## wherein N represents the overall molar ratio of reactant alkylene oxide to reactant alkanol, n represents alkylene oxide adduct number, P(n) represents the mole percent of alkoxylate product molecules having the adduct number n, and e indicates the natural logarithm function. In effect, this expression reflects a reaction mechanism under which all hydroxyl-containing species in the alkoxylation reaction mixture (i.e., both alkanol reactant and alkoxylate intermediates) react with the alkylene oxide at the same rate.
Although acid catalysis provides a relatively narrow distribution product, it is known to have substantial disadvantage in several other respects. For instance, the acids, are often unstable with limited life and effectiveness as catalysts in the alkoxylation mixture. Both the acid catalysts themselves and their decomposition products catalyze side reactions producing relatively large amounts of polyalkylene glycols, and also react directly with the components of the alkoxylation mixture to yield undesirable, and often unacceptable, by-products such as organic derivatives of the acids. Overall, use of acid alkoxylation catalysts is known to result in relatively poor quality products.
Also of substantial importance in the alkoxylation of the higher (C.sub.6 to C.sub.30) alkanols is the ability of the process to minimize the quantity of unreacted (or residual) alkanol reactant remaining in the product. A high level of residual alkanol either represents a loss of valuable reactant, or requires that further processing of the product be carried out to recover the alcohol. Moreover, the presence of unreacted alkanol is recognized to be of disadvantage from the standpoint of product quality and environmental concerns. For instance, residual alkanol in the product contributes to volatile organic emissions during spray drying of detergent formulations.
It has recently been reported in the art that, in addition to conventional acidic catalysts, a number of other substances have been found to function as catalysts or in co-catalyst combinations which are capable of producing relatively narrow distributions for the oxyalkylene adducts of higher akanols, and, in some cases, products having relatively low levels of residual alkanol reactant. For instance, it has recently been disclosed (U.S. Pat. Nos. 4,306,093 and 4,239,917, and published European Patent Applications Nos. 0026544, 0026546, and 0026547) that certain compounds of barium, strontium, and calcium promote narrow-range alkoxylation products. U.S. Pat. Nos. 4,210,764 and 4,223,164 describe the use of cresylic acids to promote alkoxylation catalyzed by barium and strontium compounds. U.S. Pat. No. 4,302,613 reports that a more peaked reaction product can be obtained by combining barium and strontium alkoxylation catalysts with co-catalysts such as calcium oxide, calcium carbide, calcium hydroxide, magnesium metal, magnesium hydroxide, zinc oxide and aluminum metal. U.S. Pat. No. 4,453,023 describes a process for preparing alkoxylates having a narrower molecular weight distribution which employs a catalyst comprising a barium compound and a promoter selected from the class consisting of superphosphoric acid, phosphoric acid, diphosphoric acid, triphosphoric acid, phosphorous acid, dihydrogen phosphate compounds, oxides of phosphorous, carbon dioxide, and oxalic acid. U.S. Pat. No. 4,453,022 describes a similar process wherein the catalyst comprises a calcium or strontium compound and a promoter selected from the class consisting of superphosphoric acid, phosphoric acid, diphosphoric acid, triphosphoric acid, phosphorous acid, dihydrogen phosphate compounds, oxides of phosphorus, sulfuric acid, bisulfate compounds, carbonic acid, bicarbonate compounds, carbon dioxide, oxalic acid and oxalic acid salts, sulfur trioxide, sulfur dioxide, and sulfurous acid. U.S. Pat. No. 4,375,564 reports that a narrow range product results from alkoxylation reactions catalyzed by a magnesium compound in combination with a compound of one of the elements aluminum, boron, zinc, titanium, silicon, molybdenum, etc. U.S. Pat. No. 4,483,941 discloses catalysts for alkoxylation reactions which comprise either BF.sub.3 or SiF.sub.4 in combination with an alkyl or alkoxide compound of aluminum, gallium, indium, thallium, titanium, zirconium and hafnium. U.S. Pat. No. 4,456,697 describes an alkoxylation catalyst which comprises a mixture of HF and one or more metal alkoxides. Japanese patent specification No. 52051307 to Tokuyama Soda KK discloses the selective preparation of mono- rather than di- or tri-alkylene glycol esters from alkylene oxide and alcohol using solid acid catalysts such as silica, alumina, titania, vandium pentoxide, antimony pentoxide, titanyl sulfate, tungstic acid, phosphotungstic acid, and silver perchlorate.